Different types of solar stills were designed and constructed by the research group, and most of the data was collected on the roof of Zheneng Chuangye Building, Hangzhou. The shape, insulation, absorption and evaporation structure of stills were improved:
Figure 1. Illustration for different solar still types
Solar stills were arranged and placed on the roof facing south direction. Distilled water was collected in a graduated cylinder to measure its volume. Distilled water was poured back into the still chamber at the sunset (17:00~18:00) or at the sunrise (8:00~9:00). Further automatic water collecting and pumping system would be introduced soon, refreshing the sea water at midnight. Solar radiation intensity and other evironmental weather data were collected by meteorological equipments located on the same roof.
Figure 2. The plot of daily solar still water production energy and solar radiation energy was shown below (daily water production was converted into its evaporation energy by multiplying water evaporation enthalpy):
Hint: It’s an interactive graph! You can slide the bottom Date range and mute/show the still data by clicking its label
Results:
Sunny day’s solar radiation energy reached high (7~8 kWh) in July and August, and so did the solar still water production energy. It rained a lot in June and September, thus small amount of water was collected in those times.
Since the weather and solar radiation were different every day, solar stills’ energy efficiencies displayed randomly by date. Thus we could differ the energy efficiency of different solar stills by daily solar radiation energy, despited of the climate and weather change.
Figure 3. The plot of solar still daily water production energy efficiency vs solar radiation energy was shown below (the loess smoothing was conducted to each solar still respectively):
Hint: It’s an interactive graph! You can mute/show the still data and its smoothing line by clicking its label.
Results:
The energy efficiency scattered widely along the “Efficiency ~ Solar Radiation” plot, which indicated there were a lot of other factors affecting the solar still’s efficiency. Here we put an example to show how the Direct Radiation and the Diffuse Radiation components in Solar Radiation would affect the solar stills’ energy efficiency. Further multiple factor analysis and simulation would be conducted by machine learning methods.
Figure 4. 3D plot of solar still daily water production energy efficiency vs direct solar radiation and diffuse solar radiation was shown below: Hint: It’s an interactive graph! You can mute/show the still data dots by clicking its label, you can also spin or expand the plot by your mouse.
Results:
According to the 3D plot, we assumed that direct radiation and duffuse radiation had different ability to improve the solar still’s energy efficiency. To confirm our assumptions, we conducted a simple multiple linear regression simulation to energy efficiency with direct and diffuse radiation.
## Intercept Direct_Coef Diffuse_Coef R_squred
## Bottom_Heating 0.1202568 0.03703141 0.04758782 0.7923695
## Sidewall_Bottom_Heating 0.1989143 0.03007121 0.03524164 0.8712875
## Bottom_Interfacial 0.1049363 0.03405803 0.04367486 0.4827847
## Foam_Bottom_Interfacial 0.2375451 0.02418384 0.05008470 0.5504512
## Sidewall_Interfacial 0.2054951 0.02702260 0.03693892 0.6170596
## Foam_Sidewall_Interfacial 0.1547691 0.04018798 0.06900478 0.8514864
According to the coefficients table, the direct radiation coefficient is smaller than diffuse ones. However in some solar stills, multiple linear model fitted not well according to their R-squreds.
We could also visualize this assumption by dividing solar radiation into three different weathers: Overcast, Cloudy, and Sunny days, which were quantified by Direct/Diffuse energy ratio as “<0.1”, “from 0.1 to 1”, and “>1”.
Figure 5. Plot array of solar stills’ daily water production energy efficiency vs solar radiation in different weather condition were shown below: Hint: It’s an interactive graph! You can mute/show the dots and lines by clicking their labels.
Results:
We also conducted hourly desalination water production collection in day time (from 8 a.m. to 5 p.m.). Sometimes the data collection interval time was two or three hours because of busy working. In cloudy days or rainy days as well as weekends, hourly water production collection was not conducted. Solar radiation intensity and other evironmental weather data were collected in minutes by meteorological equipments. Further automatic water collecting and pumping system would be introduced to collect water production collection in minutes all over the day too.
Solar energy was rising and droping from morning to evening, while desalination water production followed the same principle, however with a delay from minutes to hours. Here we normalized the solar energy and water productions over the day, showed the accumulated percentages of solar energy received and water produced in each hour for every solar stills.
Figure 6. Normalized plot of solar stills’ water production percentages and solar radiation percentages in one day were shown below, and the binomial simulation was used: Hint: It’s an interactive graph! You can mute/show the dots and lines by clicking their labels.
Results:
Hourly average data or so called transient data was calculated to show the solar stills’ dynamic performances over the day.
Figure 7. Plot of solar stills’ transient energy efficiencies in one day were shown below: Hint: It’s an interactive graph! You can mute/show the dots and lines by clicking their labels.
Results:
The peak of energy efficiency for each solar stills on the plot above was the average value at this time thoughout all days, which was not so presentative for solar stills’ performance comparison. Thus we calculated the energy efficiency vs solar power to show the dynamic performances for solar stills. Because the bottom heating solar stills were time delayed too much to present their true dynamic solar energy efficiencies, so we deleted them on the plot.
Figure 8. Plot of interfacial evaporation solar stills’ transient energy efficiencies vs solar power were shown below: Hint: It’s an interactive graph! You can mute/show the dots and lines by clicking their labels.
Results:
In traditional bottom heating solar stills, sidewall was made of steels and painted black to absorb the sunlight and transport the heat to the water layer. But on the opposite, in interfacial evaporation solar still system, according to our design and tests, sidewall needed to be thermal insulated. Although
There were still something need to discuss on the observed results.
The Solar Desalination Research Project is funded by Zhejiang Energy Group, coorperated with Zhejiang Energy Yueqing Power Station and Shanghai Jiaotong University. The project focused on development of high performance solar desalination devices on three linked research area:
The solar desalination devices were developed into three product series targeting on different market demands: